This invention relates to light-sensing control systems for use in light sensitive equipment, and more particularly to improved digital light-sensing control means capable of being fabricated on an integrated circuit.
The field of application for light-sensing control systems is quite extensive. Such systems are used, for example, in electronic digital watches to automatically set the intensity of the LED display in proportion to ambient light intensity, thereby making the LED display easy to read under a variety of lighting conditions, and saving on battery power in medium and dim ambient light. They are also used, for example, in cameras to automatically set shutter speed and/or opening in proportion to surrounding light intensity, thereby eliminating manual settings and the possibility of human error.
In the past, analog light-sensing control systems, operating on R-C time constant principles, were used. In such analog systems, at a selected time, an R-C network is initialized by discharging a timing capacitor; and at a subsequent time, the R-C network is activated by charging the timing capacitor through a timing resistor and a discrete photo resistor. As the ambient light intensity increases, the photo resistor decreases. Therefore, the charging rate of the R-C circuit is a measure of ambient light intensity. Control logic senses the charging rate and, in response, generates system control signals.
An undesirable aspect of analog light-sensing control systems is that in order to provide the required slow charging rate, the timing capacitor and the timing resistor, in addition to the light sensitive resistor, must be implemented with discrete components. This is because the slow charging rate requires a large timing capacitor and a large timing resistor, both such elements requiring too much surface area to be capable of integration onto an integrated circuit chip.
For example, an R-C time constant of 9 milliseconds requires a capacitor of 5000 pF and a resistor of 1.8 million ohms. Metal-oxide-diffusion capacitors use approximately 10 square mils per pF; therefore, a 5000-pF capacitor requires a 50,000-square mil area. In comparison, an integrated circuit chip containing the entire timekeeping circuitry for an electronic watch uses only a 20,000-square mil area.
Capacitors built from a P-N junction require approximately 1 square mil of area per pF, and therefore, use less area than metal-oxide-diffusion capacitors. But 5000-pF capacitors built from a P-N junction are still too large for practical use on an integrated circuit chip, and they also have the undesirable characteristic of varying in capacitive value as the voltage across the P-N junction varies.
Resistors are commonly implemented by utilizing a diffused zig-zag pattern. Each 200 ohms of resistance requires a surface area of approximately 0.3 mils .times. 0.7 mils. Therefore, a 9-million ohm resistor requires approximately 1890 square mils of area, which is also prohibitively large when compared to the small 10-square mil area required to implement a logic gate.
Pinch resistors utilize less area, but the ohmic value of a pinch resistor is difficult to control and typically varies by 500%. Pinch resistors are, therefore, not suitable for use in an accurate R-C timing network.
Several difficulties are created by not being able to integrate the timing capacitor and timing resistor onto an integrated circuit chip. The discrete components are much more expensive than components that are integrated onto the lightsensing controller chip. The discrete components also require extra manufacturing steps in the assembly of the control system. In addition, small light-sensitive equipment, such as that employed in an electronic digital watch, has little or no room for discrete parts.
It is, therefore, an object of the present invention to provide an improved light-sensing control system.
Another object of the invention is to provide a light-sensing controller capable of being fabricated as an integrated system having one light-sensitive resistor as its only discrete component.
A further object of the invention is to provide a light-sensing controller which does not require an R-C timing network for its operation.
It is still another object of the invention to provide a digital light-sensing control system.
A still further object of the invention is to provide an electronic timepiece which includes a digital light-sensing means.